1. Field of the Invention
The present invention is generally related to the storage of spent nuclear fuel and in particular to the consolidation and storage of spent fuel rods.
2. General Background
In commercial nuclear power plants, nuclear fuel assemblies are replaced at routine intervals as they are depleted in various locations to maintain the most efficient power level and power shaping within the core of the reactor. Due to limited or nonexistent fuel reprocessing facilities, each power plant must store these radioactive spent fuel rods on site until fuel reprocessing becomes available or the radioactivity has decayed to safe levels. Storage in water is necessary for radiation attenuation and removal of decay heat. The limited underwater storage space at commercial nuclear plants results in the need for the most efficient use of available space due to the number of fuel assemblies used during the projected life of the reactor. Efficient use of the space is achieved by removing the fuel rods from the grid assemblies and repacking the fuel rods with a minimum of spacing therebetween. This allows fuel rods from two fuel assemblies to be stored in a canister that only occupies the space that one fuel assembly would normally occupy. This consolidation ratio of 2 to 1 is achieved by packing the spent fuel rods closely together in a triangular pitch array. Patented approaches to this problem which applicant is aware of include the following.
U.S. Pat. No. 4,619,808 and U.S. Pat. No. 4,659,535 each disclose the use of a tapered transfer canister that receives the rods in a loosely packed rectangular array and rearranges the rods into a relatively tightly packed rectangular array at the lower end of the canister. The rods are then transferred to a storage container from the consolidation canister.
U.S. Pat. No. 4,659,536 and U.S. Pat. No. 4,683,110 disclose the use of flutes, springs, and a moving installation sheet for tightly packing rods in a triangular pitch array.
U.S. Pat. No. 4,636,351 discloses the use of a dense-packing device having clamping means for compressing the introduced fuel rods into a hexagonal array.
U.S. Pat. No. 4,441,242 discloses a consolidation system having a row ordering section for rearranging the configuration of the fuel rods, a horizontal consolidation section for horizontally compacting several rows of fuel rods, and a vertical consolidation section for vertically compacting several rows of horizontally compacted fuel rods.
U.S. Pat. No. 4,723,359 discloses a horizontal consolidation system that pivots a spent fuel assembly into a horizontal position and then pulls the fuel rods out into a horizontal position at a second station where they are reconfigured in the original square pitch formation into a more compact array.
U.S. Pat. No. 4,474,727 discloses a storage container for spent nuclear fuel rods having steps that support the fuel rods in a triangular array with the assistance of a movable guidance plate.
As seen from the above approaches, removing fuel rods in the square array in a fuel assembly and repacking them into a triangular pitch array is complicated by the fact that, when performed in a vertical orientation, gravity provides little or no benefit to packing the rods in a triangular pitch array. Performing these operations in a horizontal orientation is not always practical due to limited space in spent fuel pools. When loading a storage or transfer canister in the vertical orientation, the fuel rods must be guided and held in position to insure that they are held in the proper packing array until the canister is fully loaded. Difficulties in maintaining the close pack array also arise when there is a need to remove leaking fuel rods from a canister or there are insufficient rods to fully load a canister. The approaches above present large mechanical systems which are maintenance intensive due to the need to operate underwater and become increasingly more difficult to maintain the longer they are used due to contamination buildup. There exists a need for a fuel rod consolidation structure that occupies a minimum of space, that maintains rods in a triangular pitch array when only partially loaded or when a fuel rod is removed, and that requires little or no maintenance.